P0141 Intermittent code on a 2018 BRZ, fully stock
 

I bought my 2018 BRZ (Blue, Perf Pack, 6MT) in April of 2020, with 25,000km. The car is my daily driver. It gets driven year-round, usually 4-5 times a week. I drive hard occasionally when I get out of the city, and have been to the track once last year, long before any problems started. Standard maintenance: she gets frequent oil changes, never less than 91 octane, and I never drive hard until she's warmed up.

It had been running just fine making happy sewing machine noises until last October (2021). Cruising on the highway, CEL came on suddenly. Car was warm and had been running fine for ~40 minutes. I pulled over and read the code:
P2096 - Post Catalyst Fuel Trim System Too lean, Bank 1

I booked the car in to a local dealer, because this kind of problem was way over my head. A few days before the appointment, the CEL cleared itself. I cancelled the appointment and waited, knowing damn well the code would return. Fast forward a few weeks, the code returns while driving again. The dealer took a look at it and told me the downstream O2 sensor (Bank 1 Sensor 2) needed to be replaced. I replaced it myself, given that it was not difficult to do.

After replacing the sensor, it ran fine for about a month. Some time in December, the CEL returned. It disappeared before I got a chance to read it. Then it came back a few days later:
P0141 O2 Sensor Heater Circuit Bank 1 Sensor 2

It disappeared shortly after, and returned about two months ago (April 2022). Ever since then, the behavior has been very consistent. When you start the car and its cold, there is a 50% chance that the car throws P0141. The code does not come on while driving. It only seems to activate/clear if the car is parked long enough to cool off.

The only reason I haven't gone back to the dealer yet is because the O2 sensors are not covered under warranty. So I'd like to know if there is any other low-cost options I can try before I return to them.

* Does anyone have any ideas what might be wrong?
* Should I just go back to the dealer?
* Is there something I can test on my own?
* Could this be a wiring harness problem rather than an O2 sensor problem?


Thanks!

So OEM tune?

Seems either a bad connection/cable/electric short, or damaged Rear o2 sensor, check also the correlated fuse that should be EFI HTR
Also would be good to have some logs somehow

Yes, OEM tune as far as I know (I'm the 2nd owner). It has stock headers, and I have no reason to believe it's been tuned. Thanks for the tip about the fuse, I will check that today. Maybe it's loose?

I think I can get some logs with my bluetooth code reader. I'll give that a shot and post here later. I've doubted the O2 sensor was bad, but it is possible I got a faulty replacement sensor.

Or .. sensor was damaged during installation.